1. Field of the Invention
The present invention relates generally to the operation of a mass flow controller (MFC) or a mass flow meter (MFM).
2. Discussion of the Related Art
Many industrial processes require precise control of various process fluids. For example, in the semiconductor industries, mass flow controllers are used to precisely measure and control the amount of a process fluid that is introduced to a process chamber. The term fluid is used herein to describe any type of matter in any state that is capable of flow. For example, it is to be understood that the term fluid may apply to liquids, gases, vapors, and slurries comprising any combination of matter or substance to which controlled flow may be of interest.
The maximum flow rate measured by a typical thermal mass flow sensor as utilized in a mass flow controller or a mass flow meter is typically limited to approximately 10 sccm depending on the sensor design. Consequently, all MFCs and MFMs utilize a flow bypass so the device can obtain higher flow rates. The perfect bypass would maintain a constant ratio. In other words, total flow for a MFC with a 10:1 flow bypass ratio would be 11 sccm when the sensor is reading 1 sccm and 110 sccm when the sensor is reading 10 sccm. Unfortunately, bypass ratio is affected by the bypass design, fluid properties and the flow path upstream and downstream of the sensor/bypass assembly. So the bypass ratio will change as the flow rate is increased. The change in flow ratio is commonly referred to as “bypass non-linearity.” Such non-linearity degrades the instrument range or turndown and therefore the instrument accuracy. High flow bypass designs typically exhibit extremely poor linearity.
Accordingly, the disclosed embodiments seek to provide one or more solutions to the above problems. In particular, the inventors determined that most of the non-linearity is due to entrance affects at the bypass inlet. Thus, the inventors recognized that any improvement in bypass linearity is valuable because it will improve accuracy on process gas and improve the range of the instrument.